185/80R17 vs 185/85R17
Going from one to the other changes the overall height by +2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 185/80R17 | 185/85R17 | Change |
|---|---|---|---|
| Overall height | 28.7" | 29.4" | +2.5 % |
| Sidewall | 5.83" | 6.19" | +6.3 % |
| Section width | 185 mm | 185 mm | 0.0 % |
| Circumference | 90.0" | 92.3" | +2.5 % |
| Revs per mile | 704 | 686 | -2.5 % |
| Axle height | 14.33" | 14.69" | +9.3 mm |
| Approved rim width | 5–6.5" | 5–6.5" | overlap 5–6.5" |
Both are approved on a rim 5" to 6.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 185/80R17. Fit 185/85R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,254 on the road, so it under-counts by 254. At a true 70 mph the first turns 821 times a minute, the second 801.